Insulation monitoring system for insulated high voltage apparatus

ABSTRACT

A non-ferrous current sensor is used to continuously measure the charging current magnitude in step-graded foil and paper insulation systems and capacitive insulation systems used on high voltage measurement and control apparatus such as instrument transformers and on the condenser bushings of power transformers and circuit breakers. The low voltage measured signal is conditioned by an electronic circuit such that it modulates a DC signal in a system current control loop to provide continuous remote monitoring and alarm functions. Remote power is supplied to the sensor and conversion circuits through the current loop. The sensor detects changes in the charging current through the insulation ground conductor, indicating the degradation of the dielectric properties between the foil layers. The sensor and associated electronic circuitry are enclosed in a housing adaptable to be used with a capacitance tap common to high voltage measurement and control apparatus.

TECHNICAL FIELD

Applicant's invention relates generally to a system which monitors andmeasures the insulation status with respect to earth ground of a highvoltage electrical network and more particularly to a system thatquantitatively measures the capacitively coupled charging current in astep-graded paper and foil or capacitive insulation system to produce asignal output proportional to the current for continuous monitoring andalarm level detection to indicate the possibility of failure for theentire insulating system.

RELATED APPLICATIONS

This application is related to the following, commonly assignedapplication filed on Sep. 27, 1993, U.S. application Ser. No.08/127,207, U.S. Pat. No. 5,471,144 granted on Nov. 28, 1995, entitled"SYSTEM FOR MONITORING THE INSULATION QUALITY OF STEP GRADED INSULATEDHIGH VOLTAGE APPARATUS." The contents of this application are expresslyincorporated herein by reference.

BACKGROUND ART

Step-graded foil and paper insulation systems and capacitive insulationsystems are generally employed on high voltage measurement and controlapparatus such as current transformers for the purposes of protectingpersonnel from shock hazard and electrical instrumentation fromequipment damage. An example of a step-graded system comprises multiplealternating conductive and dielectric layers, with the conductive layerof least potential being earth grounded. The alternate layers are usedto form an effective series capacitive divider circuit between the highvoltage conductors and ground potential. These alternate layers areusually made from foil and paper. The paper dielectric is usually oilimpregnated and is generally used in oil-filled instrument transformers,power transformers, condenser bushings and other apparatus for highvoltage electrical power systems. Some SF6 gas insulated systems use ametallized film type of capacitive insulation.

Most step graded insulation and capacitive insulation systems aredesigned such that the capacitance of each pair of alternate layers isequal, thus producing an equal voltage stress on the dielectric betweeneach conductive layer when the apparatus is energized at high voltage.In designs where each layer is of equal capacitance, the totalcapacitance of the insulation system is equal to the layer capacitancedivided by the total number of layers. A charging current through thecapacitive circuit exists and is directly proportional to the product ofthe line voltage, the line frequency, and the total capacitance. Withthe line voltage and frequency relatively constant, changes in theinsulation charging current are due primarily to a degradation in theinsulation system. Electrical breakdown between layers results indegradation of the oil purity which leaves carbon deposits, providing aconductive path which effectively constitutes a short circuit betweenadjacent foil layers. The total capacitance of an insulator exhibitingsuch degradation increases as the effective number of layers is reduced.This increase in total capacitance will increase the charging current.Furthermore, each of the remaining layers is subjected to an increase involtage stress. Ultimately, as additional layers break down, theresidual voltage-stress between the remaining layers may exceed safeoperating levels, leading to the eventual, often catastrophic, failureof the entire insulation system.

Conventional high voltage measurement and control equipment which employfoil and paper step-graded or capacitive insulation offer no inherentmeans for monitoring the insulation charging current. Methods have beendeveloped for monitoring the condition of the insulation apparatus. Mostof them employ off-line methods. A power factor test requires that thesystem be energized with a test voltage and changes in the measuredpower factor or capacitance over time are recorded to see if there areany significant changes that would indicate a shorted layer. Partialdischarge methods are effective in detecting these changes, but must beperformed off-line and may not be practical in installations whereinterruption of service is not economical. Another method, gas-in-oilanalysis, requires an oil sample to be drawn and tested to determine thepresence of various gas that are generated when the apparatus overhears,usually indicative of a breakdown of the insulation. Some other priorart systems employ a measuring resistor in series with the ground loopand measure the voltage generated by the leakage current. However,direct measurement of this voltage is often misleading due to the lackof compensating networks to overcome the influence of the capacitance ofthe insulation and effects of electrical interference. Sensing theinsulation charging current may not be satisfactorily accomplished bymeans of a resistive series element in the grounded electrode or bymeans of a ferrous magnetic core device. In either the resistive orferro-magnetic sensing method, the capacitive nature of the insulationcircuit between the high voltage conductor and ground is disturbed by- aresistive or inductive sensor to the point where the magnitude of theinsulation current is altered. Other methods inject a current at a lowerfrequency than the network and detect the resultant current flow in theeffective leakage resistance and capacitance. These methods, beingapplied off-line, are incapable of continuously monitoring for a changein the insulation charging current while the apparatus is in operation.Further, they are often intrusive to the hermetically sealed insulationcommon to these types of insulation systems.

Commonly assigned U.S. Pat. No. 5,471,144, describes an on-lineimprovement over these common methods for monitoring the quality ofelectrical network insulation. In this system, a remote sensing coilproduces a voltage output that is linearly proportional to theinsulation charging current and a remote, self powered electroniccircuit coupled to the sensor modulates a DC current control circuitproportionally to the output voltage of the sensor. An electroniccontrol circuit provides a suitable voltage supply for the modulatedcurrent and alarm threshold detection circuits within the controlcircuit compare the output proportional voltage with predeterminedlevels. Although this system provides accurate results, installation ofthe monitor is such that the sensing coil may be mounted under oil inthe transformer tank with the transmitter being located in a secondarybox of the high voltage apparatus. This further requires an air-oilfeed-through to pass wiring between the sensor coil and the transmitter.The insulation monitor becomes dedicated to the transformer and can notbe used to monitor another transformer without extensive downtime andeffort. Retrofitting an existing installation also requires a shutdownof the high voltage apparatus. Since the coil is mounted within theinsulating oil, the unit has to be removed from the installation site,drained and dismantled before the coil can be installed. The unit has tobe refilled under vacuum and the insulating oil reprocessed to removeimpurities and moisture. This is time consuming and not very costeffective.

A capacitance tap is an existing electrode provided on all condenserbushings used on high voltage power transformers and circuit breakersand is also used on current transformers. The tap provides access to theinsulation capacitance for off-line testing purposes and could be usedfor measuring voltage on-line. It would be an advantage to have anapparatus that couples directly to this tap for monitoring theinsulation quality of the high voltage equipment. This will allow onsiteretrofitting of existing installations without requiring dismantling anddraining of the insulating oil from the equipment.

SUMMARY OF THE INVENTION

Accordingly, the principal object of the present invention is to providean apparatus for the continuous, on-line, conversion of the charging orleakage current of a step-graded or capacitive insulated high voltageapparatus to a signal proportional to the leakage current.

The further objective of the invention is to provide an apparatus forgenerating the signal proportional to the charging current with a meansof installing on the high voltage apparatus without removing the highvoltage apparatus from service.

Yet a further objective of the invention is to provide a method andapparatus for providing a remote location for the conversion apparatuswith respect to the comparison apparatus.

Another objective of the invention is to provide a method and apparatusfor monitoring the leakage current of single phase and the leakagecurrents of polyphase high voltage systems.

In the preferred embodiment of the invention, the invention is comprisedof a system of essential elements including, but not limited to, aremote sensing coil producing a voltage output linearly proportional tothe insulation charging current, a remote, self powered electroniccircuit coupled to the sensor which modulates a DO current controlcircuit proportionally to the output voltage of the sensor, and anelectronic control circuit providing a suitable voltage supply for themodulated current and alarm threshold detection circuits.

The sensing coil described by the present invention utilizes a lowpermeability core, which may be in the form of a toroid, wound with ahigh number of turns to create a low inductance linear coupler fromwhich an output voltage signal is produced which is proportional to thecurrent in the grounded conductor passing through the center of thetoroid. The linear coupler also serves to electrically isolate thesensor electronic circuits from the insulation grounding system.

The monitoring system uses a signal current modulation scheme whichincludes a 4 milliampere (ma) offset zero from which electrical power isderived for the remote electronic circuits and which serves to indicatethat the electronics are functioning properly, even in the absence ofinsulation current. The zero to full scale modulation of 4 to 20 ma isused to conform to existing standards for auxiliary monitoringinstruments, indicating meters, annunciators and alarm devices which maybe series connected to the control current loop for additionalsupervisory and reporting capability. The midpoint between 4 ma and 20ma is selected to represent the insulation current at its expected levelfor normal power system voltage and total insulation capacitance. Thatis, when the power system is energized at its nominal operating voltageand the total insulation capacitance has not been degraded from itsintended initial value, the insulation charging current, will be equalto a nominal value for which the electronic circuits will cause anadditional 8 mA to flow in the control current loop thereby causing atotal of 12 ma in the loop. The electronic circuit is adjusted such thatthis same insulation charging current will produce a full scale controlloop current reading of 20 ma when the insulation current reaches avalue equal to twice its nominal value. Due to the proportionality ofthe power system voltage and the insulation capacitance to theinsulation charging current, a doubling of the insulation chargingcurrent at nominal power system voltage would indicate that the totalinsulation capacitance has achieved a level of twice its nominal value,indicating that one half of the foil and paper layers in the insulationsystem have become ineffective. Alarm system thresholds may then be setbetween the 12 ma and 20 ma control loop current levels to indicate thedegree of insulation breakdown that can be tolerated before furthermeasures are taken to investigate the condition of the insulationsystem.

The system controller, not an object of the present invention, includesa DC voltage source from which the remote sensor electronics powersupply and the modulated control loop current are derived. Thecontroller may also include alarm circuits, possibly including timedelays, whereby alarm thresholds may be established and alarm contactsmade to transfer when the control loop current exceeds predeterminedadjustable limits.

In the preferred embodiment of the invention, the remote sensing coiland the monitoring system are housed in an enclosure which readilyadapts to a standard capacitance tap usually provided on high voltageequipment. The enclosure assembly replaces the standard capacitance tap(cap-tap) cover used to ground the insulation system at the tap when itis not otherwise being used for test purposes.

Other features and advantages of the invention, which are believed to benovel and nonobvious, will be apparent from the following specificationtaken in conjunction with the accompanying drawings in which there isshown a preferred embodiment of the invention. Reference is made to theclaims for interpreting the full scope of the invention which is notnecessarily represented by such embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the construction of a step graded foil and paperinsulator showing the alternating layers of foil and paper.

FIG. 2 is an electrical schematic of a capacitive divider circuit formedby the insulation structure of FIG. 1.

FIG. 3 is a block diagram of the essential elements of a monitoringsystem utilized in the present invention.

FIG. 4 is a typical installation of the monitoring system of FIG. 3 usedon an instrument transformer according to prior art.

FIGS. 5a and 5b are a detailed block diagram of the monitoring system ofthe present invention as shown in FIG. 3.

FIG. 6 is a typical installation of the monitoring system of FIG. 3 usedon an instrument transformer according to the present invention.

FIG. 6A is a typical installation of the monitoring system of FIG. 3used on a high voltage circuit breaker bushing according to the presentinvention.

FIG. 7 is a cross sectional view of an enclosure for housing the remotesensor and transmitter illustrated in FIG. 6 of the present invention.

FIG. 8 is a detailed schematic diagram of the remote sensor andtransmitter illustrated in FIG. 7.

FIG. 9 is a cross sectional view of an alternate enclosure for housingthe remote sensor and transmitter illustrated in FIG. 7 of the presentinvention.

DETAILED DESCRIPTION

Although this invention is susceptible to embodiments of many differentforms, a preferred embodiment will be described and illustrated indetail herein. The present disclosure exemplifies the principles of theinvention and is not to be considered a limit to the broader aspects ofthe invention to the particular embodiment as described.

FIG. 1 shows the typical construction of a step-graded paper and foilinsulation system 1 where a high voltage conductor 2 is wrapped withalternating layers of dielectric material such as paper 4 and conductivematerial such as foil 6. Thus, as the conductive layers are added to theinsulation system, the radius, R0-R4, from the high voltage conductor tothe conductive layers tends to increase. The capacitance value betweenany two adjacent conductive layers is directly proportional to thelength of the layer and inversely proportional to the LOG ratio of theoutside to inside radius of the layer. In order to maintain equalcapacitance values from layer to layer, the length of each conductivelayers is adjusted to account for the radial buildup. This constructionserves to insulate the outermost layer 8, usually at ground potential,from the high voltage conductor 2.

A simplified electrical schematic diagram of the insulation system 1 asit forms a series capacitance circuit from the high voltage conductor 2to ground 10 is shown in FIG. 2. With the number of capacitors equal tothe total number of foil layers, a capacitive voltage divider 12 isformed. The total capacitance of the series capacitors, with equalvalued capacitors, is equal to the layer capacitance divided by thenumber of layers. The actual equivalent circuit will consist of manyparallel capacitors created by the capacitances that exists betweennoncandidate layers. The electrical current 14, also called the chargingcurrent, which flows through the capacitor circuit is proportional tothe voltage of the high voltage conductor 2 and the total capacitance12, based on the relationship that ##EQU1##

On a typical 345 kV high voltage current transformer, the totalcapacitance may be approximately 705 pf. Operating on a 345 kV line, theline to ground voltage is about 200 kV and the charging current 14 isnominally 53 ma. Under normal operating conditions, the high voltage ACsignal 2 has a relatively constant amplitude and frequency. Thus, thecharging current 14 remains relatively constant in amplitude andfrequency as well. Changes in the capacitive insulation structure,however, will produce changes in the amplitude of the charging current.Using the circuit shown in FIG. 2, which represents a four layer system,as an example, a voltage breakdown between two adjacent foil layers willeffectively reduce the total number of layers by 1, which in turnincreases the total capacitance 12 by a factor of 1/3. This will cause aproportionate increase in the charging current 14 to over 70 ma.Although FIG. 2 only shows four layers, more typical systems will use 10to 30 layers, depending on the voltage class of the unit.

FIG. 3 illustrates an entire insulation current monitoring system 18 ina block diagram as disclosed in commonly assigned U.S. application Ser.No. 08/127,207. A step-graded foil and paper insulation system 20insulates a high voltage conductor 22 from a transformer core and coilassembly 24. The step-graded insulation system includes alternate layersof dielectric material such as paper 23 and conductive material such asfoil 25. The lowest potential foil layer 26 is electrically connected toground 10 with a conductive element 30 which provides a ground path forthe insulation charging current 32. A wound, non-ferrous toroidal coilassembly 34 is positioned such that the ground conductor 30 passesdirectly through the center of the toroidal coil, such that the coil 34links all of the magnetic flux generated by the charging current 32passing through the conductor 30. The non-ferrous nature of the coilassembly 34 results in a low inductance coupling back into the seriescapacitive circuit of the insulation 20 and, therefore, does not alterthe magnitude of the charging current 32 being measured. A burdenresistor 35 internal to the assembly 34 produces a voltage Vs that isproportional to the charging current 32. The coil assembly 34 alsoserves to electrically isolate the current monitoring system circuits 18from the insulation grounding system. Because of the high voltage andcurrents associated with the system, the coil assembly will have sometype of shielding to prevent inductive coupling of adjacent and unwantedmagnetic fields.

The coil assembly 34 is electrically connected to a sensor electroniccircuit or transmitter 38 by means of a shielded, twisted pair, or othersuitable, cable 36. The transmitter 38 performs the functions ofamplifying and rectifying the sensor voltage signal Vs. Output drivercircuits in the transmitter 38 are used to modulate a DC current I1 in acontrol loop 40. The modulation results in the current signal I1containing a proportionate magnitude of the charging current 32. Thecontrol loop 40 conforms to a standard 4-20 ma loop and cable 42 whichelectrically connects the transmitter 38 to a controller 44 is also ashielded, twisted pair or other suitable cable. The controller 44 may belocated in a benign control house 43 environment. The control house 43and the transmitter 38 can be separated by up to 2000 feet.

Controller 44 performs one or more essential functions. The controller44 includes an isolated DC voltage source, which may provide as much as30 to 40 volts, and which has the capability of providing 20 ma fullscale modulated current in the control loop 40. The current which flowsfrom the voltage source is strictly modulated by the sensor electronicsalone, yet the terminal voltage across the controller 44 output isdetermined by a nominal voltage level required to excite the remotepower supply circuits in the transmitter 38, and by the voltage dropsassociated with the current in the control loop 40. The outputs of thecontroller 44 are alarm contacts 45 which operate at a predetermined,settable level of leakage or charging current 32 to indicate a change inthe insulation charging current 32 of the insulation system 20. If apolyphase system is being monitored, the controller 44 is capable ofreceiving inputs from transmitters and control loops similar totransmitter 38 and control loop 40.

The insulation current monitoring system 18 is powered by control power46 which is inputted to the controller 44. The signal current modulationscheme includes a 4 ma offset zero from which electrical power isderived for the remote transmitter 38, eliminating the need forauxiliary power at the high voltage apparatus that is being monitored.

FIG. 4 shows a typical installation of a prior art insulation currentmonitoring system. An instrument transformer 47, utilizing a step gradedfoil and paper insulation system, is shown with tank wall 48 at groundpotential and the grounded lead 30 from the insulation system broughtthrough the tank wall 48 and the sensing coil 34 before beingelectrically grounded to the tank wall 48 itself. The sensing coil 34 isshown outside the tank wall for clarity. More typically, the sensingcoil 34 is located inside the tank 48, immersed in oil.

The insulation charging current 32 which flows through conductor 30 issensed by the sensing coil 34, whose output signal Vs is then coupled bycable 36 to transmitter 38 so as to modulate the current I1 in thecontrol loop 40 coupled by cable 42 to the controller 44 in the controlhouse 43. Transmitter 38 is installed internally to the instrumenttransformer 47 within a secondary box 39. The controller 44 providesremote power to the sensor electronics and monitors the charging currentlevels in the control current loop 40 as previously described. Only asingle phase system is shown and would be typical of a retrofit.

Operation of the insulation current monitoring system 18 can be bestunderstood with reference to FIGS. 5a and 5b, which are detailed blockdiagrams of the preferred embodiment of an insulation current monitoringsystem 18 employed in the present invention. The foil layer 26 of theinsulation system being monitored is coupled to ground 10 through groundlead 30 which passes through the sensing coil 34. The output voltage Vsof sensor 34 is proportional to the insulation charging current 32 andis coupled to the transmitter 38. For the low leakage currents beingmeasured, it has been found that a proportionality factor or ratio offifty microvolts per one milliampere of charging current provides anadequate degree of sensitivity. A combination filter, amplifier andrectifier circuit 54 develops a DC voltage signal 56 from voltage Vsthat remains proportional to the charging current 32. A voltagecontrolled current source 58, where the generated current isproportional to the voltage 56, is used to modulate a 4-20 ma currentloop 40 for inputting to channel A of controller 44. Controller 44 isshown for a three phase system, but with only phase A sensor 34 andtransmitter 38 shown. Additional sensors and transmitters would berequired for a three phase system.

The current I1 flowing in the current loop 40 is coupled to thecontroller 44 via a twisted pair cable 42 as previously detailed. Thecurrent controlled current source 62 provides 4 ma of quiescent currentflowing in loop 40 with zero charging current 32 when the high voltageapparatus being monitored is not energized. This quiescent current isused to provide power for the sensor 34 and transmitter 38. With thehigh voltage apparatus energized during initialization, usually duringinstallation, the voltage controlled current source 58 is adjusted toprovide 12 ma of loop current. This represents the expected chargingcurrent 32 when the high voltage apparatus is operating at its nominaloperating voltage. Thus 100% (1X) of initial charging current equals 8ma. Since there is a linear relationship between the charging current 32and the loop current I1, 200% (2X) of charging current will equal 2×8 maor 16 ma which is added to the 4 ma of quiescent current to create 20 maof loop current I1. A current mirror 64 generates a voltage V1 acrossresistor 66. V1 is calibrated to be proportional to the charging current32.

A 2X threshold level generator 68 and an adjustable 1.2-2X thresholdlevel generator 70 generate voltage levels V2, V3, respectively. V2 isset such that voltage V1, with 20 ma of loop current I1, will equal V2.V3 is adjusted within the range 1.2-2X or 13.6-20 ma of loop current I1for a similar relationship with V1. Schmidt trigger 72 compares V1 withV2 and will enable delay counter 1 when V1 exceeds V2, indicative of anincrease in the charging current 32 to a level that is at least twicethe original current resulting from a breakdown in the insulating layersof high voltage apparatus. Schmidt trigger 74 compares V1 with V3 in asimilar fashion.

Since switching transients may regularly occur on the power system,voltage levels on the power system will momentarily cause the insulationcharging current 32, and hence voltage V1, to increase above theirsteady state levels when such transient conditions occur. Delay counters1-6 provide a predetermined delay period before initiating the transferof the alarm contacts 50a,b. This distinguishes between a short termoccurrence of a switching voltage transient and a long term steadydegradation of the insulation system as evidenced by a steady increasein the insulation charging current. Clock 75 produces a timing pulse 76for the delay counters 1, 3, and 5, with a divide by ten circuit 77providing timing pulse 78 having a longer time delay for counters 2, 4,and 6 that monitor the lower levels of charging current 32 in the 1.2-2Xrange. The clock rate for the 2X alarm level is adjustable and canprovide up to 30 seconds of delay.

If the charging current 32 exceeds the 2x or adjustable thresholds 68,70 for a period longer than the time duration of the delay counters 1-6,outputs 79 or 80, depending on which threshold was exceeded, willenergize latching switches 81, 82 respectively. This will in turnenergize the 2X or 1.2-2X alarm contacts 45a or 45b respectively throughone of the "OR" functions 84, 86. Alarm indicators 88a or 88b willdisplay the appropriate cause of the trip. The outputs will remain in atripped state until a master reset 90 is operated. Latching switches 81,82 are used to provide memory of the condition if control power 46 isinterrupted at any time after a trip operation.

Channels B and C will perform in the same manner. OR 84 will operate the2X alarm if any of the three phases exceeds the 2X level for the presettime delay period and OR 86 will operate the 1.2-2X alarm if any of thethree phases exceeds that level for the other preset time delay period.Power for the system is supplied by a switchmode regulator 92, thedetails of which are well known and are not an object of the presentinvention.

Most high voltage insulation systems, such as those used in currenttransformers and on all condenser bushings used on power transformersand circuit breakers, have a capacitance tap provided on all condenserbushings. The capacitance tap is an electrode that provides access tothe insulation capacitance for off-line testing and is also occasionallyused to measure voltage on-line. A cover protects the capacitance tapand is used to ground the insulation system at the capacitance tap. Thiscover is more commonly known as a cap-tap cover. The tap connects to thesecond to last conductive layer of the insulation system and theoutermost conductive layer is grounded internally. This creates acapacitive divider between a capacitance 01 from the tap to the highvoltage conductor and a capacitance C2 between the tap and ground.Knowing the ratio, C1 to C2, tests can be performed on the high voltagesystem by applying a known voltage to the tap. During normal operation,the cover shorts out the capacitance C2.

The present invention is an improvement over the prior art in that thesensing coil 34 and transmitter electronics 38 are enclosed in housing100, which also serves to ground an insulation capacitance tap 102, justas the cap-tap cover previously described, and as shown in FIG. 6. Thishousing is used in place of the cap-tap cover and is coupled to thecapacitance tap 102 of the instrument transformer 47, which utilizes astep graded foil and paper insulation or capacitive insulation system.The tank 48 is at ground potential 10 and the grounded lead 30 from theinsulation system is brought through the tank wall by capacitance tap102 into the housing 100 and through an internal sensing coil 34 beforebeing electrically grounded to the tank 48 itself. The insulationcharging current 32 which flows through conductor 30 is sensed by thesensing coil 34, whose output signal Vs is then coupled to transmitter38 so as to modulate the current I1 in the control loop 40 coupled bycable 42 through pull box 92 to the controller 44 in the control house43. Pull box 92 is grounded to ground 10 and provides a means forterminating the shield 94 of cable 42. Transmitter 38 and sensor 34 areboth installed internally to Cap-Tap cover housing 100. The controller44 provides remote power to the transmitter electronics 38 and monitorsthe charging current levels in the control current loop 40 as previouslydescribed. Only a single phase system is shown and would be typical of aretrofit. In addition, FIG. 6 shows the capacitance tap 102 as part ofan instrument or current transformer. It could also just as easily bepart of a circuit breaker 50 having condenser bushings 52 as illustratedin FIG. 6A. The housing 100 containing the sensor 34 and transmitter 38would be adaptable to connecting to any capacitance tap. The bushing 52is provided with the capacitance tap 102 and may be connected to aconcentric metal electrode or capacitance divider inside the bushing. Asan alternative construction, alternating layers of dielectric material,such as paper, and conductive material such as metallized foil are woundaround the conductor as shown in FIG. 1. The bushing 52 will have aground sleeve for grounding to the circuit breaker tank or housing 54.

A cross sectional view of the housing 100 for enclosing the sensing coil34 and transmitter 38 is illustrated in FIG. 7 of the present invention.A flange 104 is welded to the tank wall 48. The flange 104 containstapped holes 106 for securing and sealing the capacitance tap 102 to theinstrument transformer 47 or other types of high voltage equipment bymounting plate 108 and bolts 110. Capacitance tap 102 consists of anelectrode 112 which is coupled to the grounded lead 30 from theinsulation system and is isolated from the tank wall by isolationbushing 114 and associated filler material 116. This type of capacitancetap is commonly referred to as a "type A" tap. Coupling a monitoringdevice to the capacitance tap 102 is normally accomplished by screwingthe device into threads 118. Adaptor 120 uses these threads 118 tosecure the housing 100 to the instrument transformer 47. Internal to thehousing 100, a threaded cylindrical rod 122 is attached to housing 100and is encircled by the sensing coil 34 and provides the means forsecuring a circuit board 126 containing the transmitter 38. Alsoattached to rod 122 are magnetic shields 125, 126 for shielding sensingcoil 34 and transmitter 38 from potential electrical and magnetic fieldsgenerated within and around the instrument transformer 47. Screw 128mounts a spring contact 130 to complete the assembly of housing 100. Anaccess hole 126 provides a feedthrough for cable 42 which connects thetransmitter 38 to the controller 44 through connector 129.

Previously, when a monitoring device was not attached to the capacitancetap 102, a tap cap cover consisted of a housing with a spring contactattached for mating with the electrode 112. This provided a ground pathfrom the outer conductive layer of the insulation system. With thepresent invention, rod 122 mates with spring contact 130 when thehousing 100 is attached. Thus, the electrical path for the groundcurrent is from the outer conductive layer or foil of the insulationsystem to the ground lead 30, through the electrode 112 and springcontact 130, through the screw 128 and rod 122, and into the housing100, which is grounded to the tank wall 48 by the adaptor 120 and thecapacitance tap 102. The sensing coil 34 therefore senses all groundcurrent from the insulation system.

FIG. 8 details the sensing coil 34 and transmitter 38 consisting of therectifier circuit 54 and voltage controlled current source 58. Currentloop 40 is connected to terminals J-1 and J-2. The 4 ma quiescentcurrent of the loop current I1 creates a 6 volt rail between 132 and 134through the action of zener diode VR1 and transistors Q2a, b, and c.Four ma of current flowing in the current loop 40 is sufficient toprovide base drive at node 136 for dartington connected transistors Q2band c. Transistor Q2a will conduct, allowing zener diode VR1 to alsoconduct, building a voltage at node 136. Regardless of any increase inthe loop current, the rail voltage will remain relatively constant andregulated near the 6 volt level.

The output voltage Vs of sensor 34 which is proportional to theinsulation charging current 32, is inputted to rectifier circuit 54.Changing the resistance values of resistors RA and RB provide a meansfor using the insulation current monitor 18 for different adjustmentranges of charging current 32, based on the magnitude of the highvoltage line and the total capacitance of the insulated apparatus thatthe insulation current monitor 18 is monitoring. These resistors, alongwith potentiometer RV1, scale the incoming voltage Vs to the fixed gainof buffer amplifier 138, which can be an operational amplifier forgreater temperature stability. The gain is selected such that AC outputvoltage V4 does not saturate at the 200% level of allowable insulationcharging current 32. Since Vs is scaled to be fifty microvolts per onemilliampere of charging current, the gain of amplifier 138 has to bequite high. A two stage cascaded amplifier is employed with the firststage with amplifier 138 providing half of the required gain so as toallow the full peak to peak voltage swing of Vs to be amplified withinthe voltage rails 132, 134. The higher gain of the first stage will alsoreduce the effects of the DC offset levels of the operational amplifierson the following stages, amplifiers 140 and 142 which only will requireequal but opposite polarity gains of the amplifiers 140 and 142, and thefull wave rectification circuitry. Capacitor C1 couples voltage V4 toamplifiers 140 and 142 which function to generate voltages V5, V6respectively, that are equal, but phase shifted by 180 degrees. As aresult, resistor R16 becomes a load resistor that produces a full waverectified voltage V7, which is the DC voltage signal 56 as referenced inFIG. 5a, through the alternate conduction of transistor pairs Q1a-Q1cand Q1b-Q1d. Voltage V7 is positive with respect to the voltage rail134. RC network C4-R17 provides a DC filter for the rectified voltage V7and the C3-R15 combination provides a balance with the positive DO rail132. Calibration of the rectifier circuit 54 is accomplished byadjusting potentiometer RV2 for zero voltage across R16 with voltage Vsnot present.

The voltage controlled current source 58 portion of transmitter 38modulates the 4-20 ma current loop 40 through the action of summingamplifier 144 and the voltages at nodes 146 and 148. The voltage at node146 comprises voltage V7, which is proportional to the charging current32, a current reference signal V8 derived from voltage divider R22 andR25, and an adjustable voltage offset signal V9 derived from divider R21and RV3. The voltage at node 148 is a current sample signal V10 derivedfrom current sense resistor R26. Amplifier 144 subtracts the voltageacross R26 from the offset voltage to compensate for changes in currentloading of the transmitter circuit 38 whereby the output voltageV11=V7+V8+V9-V10. The output voltage V11 provide base drive fortransistor Q2d which modulates current I1 in current loop 40. With nosignal present at terminals J1, J2, potentiometer RV3 is adjusted toprovide 4 ma of current in current loop 40. With 100% of predeterminedcharging current, as represented by input voltage Vs, potentiometer RV1is used to adjust the gain of the amplifier stages such that the currentI1 in current loop 40 is equal to 12 ma. This will result in I1equalling 20 ma of current with 200% of charging current 32 present.200% of insulation charging current is generally considered a triplevel.

During operation of the high voltage system, it can be shown thatextremely high voltage levels can be generated when a disconnect switchsupply power to the system is operated. Arcing across the switchcontacts generate high current pulses or surges which can flow throughthe transformer insulation and the transformer ground lead 30. The fastrise times of these pulses and inductance of the ground lead aresufficient to raise the potential voltage of the transformer tank 48 tolevels well above ground 10. This voltage exceeds the breakdown voltagebetween the transmitter electronics 38 and housing 100. This could causea failure of the transmitter 38 and the current monitoring system 18.Accordingly, a bipolar surge protector VS1 connected to terminals J1 andJ2 is used to crowbar the high frequency switching surge and pass thecurrent to ground through terminal J3, which is connected to the shield94 of cable 42. Field installation of the twisted pair, shielded cable42 cain also provide a method of further reducing effects of the highfrequency switching surge. In particular, the installation makes use oftwo cascaded, twisted pair cables for cable 42. The shield 94 of thecable 42 that connects to the transmitter terminal J3 is terminated onthe transformer tank 48 and the shield of cable 42 that connects tocontroller 44 is terminated to the control house 43 ground. The twocables are coupled to each other on a terminal strip inside of the pullbox 92 located in the switchyard. The shields of both segments insidethe pull box 92 will be terminated to ground. This provides shieldingfrom both electrical and magnetical fields.

A cross sectional view of an alternative assembly 150 for enclosing thesensing coil 34 and transmitter 38 of the present invention isillustrated in FIG. 9. This type of assembly is used with a "type B"capacitance tap 152. A flange 153 is welded to the tank wall 48. Theflange 153 contains tapped holes 154 for securing and sealing thecapacitance tap 152 to the instrument transformer 47 or other types ofhigh voltage insulation equipment by bolts 155. A spring contact 156 iscoupled to the grounded lead 30 from the insulation system by bolt 158and is isolated from the tank wall by isolation bushing 160 andassociated filler material 162. A mounting adaptor 166 provides themeans for attaching the assembly 150 to the capacitance tap 152.

Assembly 150 is identical to housing 100 of FIG. 7 with the exception ofspring clip 130 and screw 128. In their place an extension probe 164 isscrewed into rod 122. Thus, the electrical path for the ground currentis from the outer conductive layer or foil of the insulation system tothe ground lead 30, through the bolt 158 and spring contact 156, throughthe probe 164 and rod 122, and into the housing 150, which is groundedto the tank wall 48 by bolts 155 and flange 154. The sensing coil 34therefore senses all insulation ground current from the instrumenttransformer 47.

While the specific embodiments have been illustrated and described,numerous modifications are possible without departing from the scope orspirit of the invention. One possible embodiment is to replace thecurrent loop 32 with a modulated fiber optic cable. The transmitterportion 38 of FIG. 6 would include the electronics to convert thecharging current 32 to an equivalent fiber optical modulated lightsignal to the 4-20 ma current loop, the details of which are well knownto those skilled in the art. The controller 44 would contain thenecessary decoder to demodulate the light signal to input to the Schmidttriggers and other related circuits previously described. An internalpower source for the transmitter circuits 38, such as a battery or ameans for deriving power from the ground current, would be requiredhowever.

We claim:
 1. An insulation monitoring system for a high voltageapparatus having a capacitance tap coupled to an insulation systembetween a high voltage conductor and earth ground of said high voltageapparatus, said insulation monitoring system comprising:a. a transmittermodule including an AC current sensor for measuring and converting acharging current flowing in said insulation system between said highvoltage conductor and earth ground to a first voltage signalproportional to said charging current and an electronic circuit meansfor converting said first voltage signal to a DC current and modulatinga first current signal by said DC current; b. an adapter for couplingsaid transmitter module to said capacitance tap; c. a transmissionnetwork coupled to said transmitter module, said transmission networkfor sending said modulated first current signal; d. a controller coupledto said transmitter by said transmission network to receive saidmodulated first current signal, said controller for demodulating saidmodulated first current signal to extract said DC current, convertingsaid DC current to a voltage proportionally to said charging current,comparing said proportional voltage with a preset voltage level, andgenerating an alarm signal when said proportional voltage exceeds saidpreset voltage level; and e. wherein said insulation monitoring systemcontinuously monitors said insulation system.
 2. The insulationmonitoring system of claim 1 wherein said AC current sensor comprises awound toroidal coil assembly responsive to said charging current in aconductor passing through said toroidal coil without disrupting thenature of a source circuit generating said charging current to bemeasured and said coil assembly further having means, responsive to saidcharging current, for developing a linearly proportional voltage signaland means for delivering said voltage signal to said transmitter.
 3. Theinsulation monitoring system of claim 1 wherein said transmissionnetwork is a 4-20 milliampere current loop.
 4. The insulation monitoringsystem of claim 3 wherein said transmitter further includes means forgenerating regulated control power from a quiescent current level of 4milliamperes from said current loop for powering its electronic circuitmeans.
 5. The insulation monitoring system of claim 4 wherein saidelectronic circuit means in said transmitter further includes means forfiltering, amplifying and averaging said first voltage signal to producesaid DC current made proportional to said charging current.
 6. Theinsulation monitoring system of claim 5 wherein said filtering,amplifying and averaging means in said transmitter includes a full waverectifier to generate a full wave rectified voltage proportional to saidcharging current.
 7. The insulation monitoring system of claim 5 whereinsaid electronic circuit means in said transmitter further includes meansfor modulating said second signal between the bounding values of 4milliamperes and 20 milliamperes in response to said charging currentwith said DC current.
 8. The insulation monitoring system of claim 7wherein said modulating means includes a comparator which monitors saidquiescent current, computes a difference between said quiescent currentfrom a constant reference value and modulates said current loop by anamount equal to said difference and said full wave rectified voltage. 9.The insulation monitoring system of claim 3 wherein said controllercoupled to said transmission network further includes means to supplypower to said transmitter through said transmission network, means toprovide for adjustable alarm threshold levels of said charging current,and means to provide for adjustable time delayed operation of said alarmlevels when said alarm threshold levels are exceeded.
 10. The insulationmonitoring system of claim 9 wherein said controller further includesmeans to couple separate transmission networks for monitoring thecharging current of polyphase high voltage systems.
 11. The insulationmonitoring system of claim 1 wherein said insulation system is astep-graded insulation system having multiple and alternate layers ofconductive and dielectric materials.
 12. The insulation monitoringsystem of claim 1 wherein said insulation system is a capacitiveinsulation system.
 13. The insulation monitoring system of claim 1wherein said high voltage apparatus is an instrument transformer. 14.The insulation monitoring system of claim 1 wherein said high voltageapparatus is a circuit breaker equipped with high voltage bushings,wherein said capacitance tap is part of said high voltage bushings.